1. Field of the Invention
The present invention relates to a refrigeration circuit for transferring heat energy between two regions. More particularly, the present invention concerns defrost of such a system having multiple refrigeration circuits wherein a single refrigeration circuit is reversed to defrost a heat exchanger of the other circuit.
2. Description of the Prior Art
In a typical vapor compression refrigeration system, various components such as a compressor, condenser, evaporator and expansion device are arranged to transfer heat energy between a fluid in heat transfer relation with the evaporator and a fluid in heat transfer relation with the condenser. In a heat pump system an outdoor coil and an indoor coil are located such that the compressor through a reversing valve may direct hot gaseous refrigerant to either coil acting as a condenser. The other coil then acts as an evaporator such that depending upon the position of the reversing valve, heat energy is either rejected or absorbed in both the indoor coil or the outdoor coil. In the heating mode of operation heat is rejected in the indoor coil acting as a condenser and heat is absorbed at the outdoor coil acting as an evaporator. The reverse is true in the cooling mode of operation wherein the heat is rejected in the outdoor coil acting as a condenser and heat is absorbed in the indoor coil acting as an evaporator.
In heat pump systems using two separate refrigerant circuits each having a compressor it is known to stage the operation of the compressors when the heat pump is either in the cooling mode of operation or the heating mode of operation whereby first one compressor and then the other compressor is energized in sequence as the total load increases. Both compressors are simultaneously operated when the load exceeds the capability of a single circuit. The operation of both the compressors in the heating mode is usually necessary to satisfy heating requirements when the ambient temperature drops below a certain level depending upon the nature of the load and other ambient and temperature conditions.
Additionally, when in the heating mode of operation, frost may form on the outdoor heat exchangers. It is conventional in the art to provide heat energy to an outdoor heat exchanger to melt this ice accumulation. Methods such as reversing the flow of refrigerant in the circuit such that hot gas is provided to the outdoor coil to melt the frost are well known in the art. Additionally, electric resistance heaters and other heat generating devices have been utilized to melt the frost on the heat exchanger. Also known in the art are nonreversed systems wherein through a valving means relatively warm refrigerant flows through the frosted coil to melt the ice formed thereon.
The present invention concerns a refrigeration system having two refrigeration circuits preferably of different capacities, wherein the outdoor coils are located adjacent to each other. In the preferred embodiment hereof the outdoor coils are cylindrical in configuration with one located inside the other. During normal operation a fan is used to circulate heat transfer fluid, typically air, through the heat exchangers in serial arrangement. When a defrost signal is generated, indicating it is appropriate to commence a defrost cycle, one of the two outdoor heat exchangers is operated in the cooling mode by switching the reversing valve such that hot gas is directed into the outdoor heat exchanger. Additionally, the fan for drawing heat transfer fluid through the two outdoor heat exchangers is also reversed such that heat transfer fluid flows first over the outdoor coil which is rejecting heat. The heat transfer fluid absorbs the rejected heat and then travels to the frosted outdoor coil to which this heat is rejected melting the ice formed thereon. Additionally, the systems are sized such that the heat energy withdrawn from the indoor coil and transferred to the outdoor coil in the cooling mode is balanced by the heat energy transferred from the frosted over coil to its corresponding indoor coil such that the enclosure to be heated does not have heat energy withdrawn therefrom to effect defrost of the outdoor coil.